1. Field of the Invention
This invention relates to stacked plate heat exchangers. More particularly, the invention relates to a stacked plate heat exchanger which accommodates three separate fluid circuits, so that, for example, two refrigerant circuits can transfer heat from a single water circuit in a more desirable and usable manner, wherein each individual refrigerant circuit comes in thermal contact with at least all but one of the water passages.
2. Description of the Prior Art
As is known to those skilled in the art, a stacked plate heat exchanger includes a plurality of plates stacked one atop another and with their surfaces shaped and spaced to form fluid flow passages between adjacent plates. The peripheries of the plates are sealed to prevent fluid leakage and inlet and outlet openings are provided and selectively sealed so that a particular fluid passes only through selected flow passages in the stack. Sealing is accomplished by brazing, soldering or similar processes, or occasionally by use of suitable shaped gaskets positioned between plates and compressed by external clamping means holding the stack together. For optimum heat transfer, counter-current flow is generally used--i.e. the fluid in one passage flows through the stack in a direction opposite to the flow of the fluid in adjacent passages.
In refrigeration applications stacked plate heat exchangers are commonly used as condensers, water chillers, air dryers, oil coolers and other devices for refrigerant to water or oil, refrigerant to air, and refrigerant to refrigerant heat transfer. For example, in a typical prior art water chiller a single circuit of refrigerant--i.e. delivered from one source, passes through alternate flow passages and a single circuit of water passes through the remaining flow passages, whereby the water and refrigerant exchange heat energy. Although such units involve two flow circuits, one for refrigerant and one for water, they are often called single circuit chillers; however, for clarity in description herein, heat exchangers will be defined by the total number of fluid circuits accommodated, e.g. if a heat exchanger accommodates one circuit of refrigerant and one circuit of water, it will be termed a two-circuit exchanger. For purposes of explanation herein, water chillers will generally be the standard for discussion, it being understood that the invention can be used for other combinations of liquid or gaseous fluids.
Numerous designs of two-circuit chillers have been developed by the prior art. Examples of several of these are disclosed in the following listed U.S. Pat. Nos.:
Shimoya, et al. U.S. Pat. No. 5,137,082; PA1 Bergqvist, et al. U.S. Pat. No. 4,987,955; PA1 Pfeiffer No. U.S. Pat. No. 4,781,248; PA1 Sacca U.S. Pat. No. 4,470,455; PA1 Armes U.S. Pat. No. 3,240,268; and PA1 Edwards, et al. U.S. Pat. No. 3,114,686.
It is to be emphasized that all these prior art devices are designed to carry only two fluid circuits, generally a single refrigerant circuit and a single water or other fluid circuit.
In many applications single refrigerant circuits are not adequate, and one or more additional circuits are required. In such multiple circuit versions of water chillers, each separate refrigerant circuit includes a separate refrigerant compressor. This arrangement provides better part load performance, lower chiller load capabilities, and improved reliability and backup if one compressor should fail. The requirement of multiple refrigerant circuits has led to the development of some prior art water chillers in which two or more refrigerant circuits act on one water circuit in the same unit; for example, traditional prior art shell and tube type heat exchangers can be fabricated with two or more refrigerant circuits flowing through different sets of tubes. One prior art stacked plate heat exchanger described as a "multiple fluid" unit is disclosed in Donaldson U.S. Pat. No. 4,002,201; however, Donaldson's unit involves two liquids and one gas such as air, and the gas flows through open spaces provided between alternate liquid-carrying pairs of plates. The Donaldson unit is in effect only a two-circuit exchanger in which the alternating flow passages have been physically separated to form a third flow passage for the third fluid, which is a gas; such flow passage for the third fluid is not an integral part of the plate stack, so strictly speaking Donaldson does not show a stacked plate heat exchanger, as that term is generally understood. The Donaldson unit is not suited to applications where all fluid circuits contain liquids, such as is the case with water chillers.
In fact, with prior art stacked plate heat exchanger technology the inclusion of two or more refrigerant circuits in a single water chiller or other heat exchanger with liquid media in all circuits has been a continuing and complex problem. Prior art stacked plate heat exchangers can be configured into a pseudo three-circuit water chiller by putting two two-circuit heat exchangers back to back with a common water circuit passing through both exchangers. In this arrangement one refrigerant circuit flows through the first exchanger, and the second refrigerant circuit flows through the second separate exchanger. This approach is adequate in some applications, but it has limitations in that only one refrigerant is in contact with the water at any point. When both refrigerant circuits are in operation the arrangement works satisfactorily, but in the majority of water chiller operations only one refrigerant circuit is operating much of the time, and in these situations the prior art arrangement causes control and potential freeze up problems. For example, when only one circuit is attempting to hold a given output temperature for the water flowing through the unit, the operating refrigerant circuit runs at a significantly lower temperature and thereby risks freezing the water which is in contact with that refrigerant in addition to causing higher compressor power requirements. Similar problems arise in alternate prior art arrangements in which one water circuit is split so that 50% of the water flows through one heat exchanger and the other 50% flows through the second heat exchanger, both parts of the water flow coming together down stream of the two exchangers; the thermal relationships are virtually identical in either prior art multiple circuit arrangement.
The problems existing with prior art attempts at three-circuit water chillers could be avoided if both refrigerant circuits were in thermal heat transfer contact with substantially all of the water flowing through the chiller.